Understanding WebSocket Protocol
1. Understanding Protocol Basics
WebSocket (RFC 6455) is a full-duplex, bidirectional TCP-based protocol over a single long-lived connection, initiated by an HTTP/1.1 Upgrade handshake. Schemes: ws:// (port 80) and wss:// (port 443, TLS).
Property Value Description RFC 6455Core protocol specification Transport TCP Persistent single connection Direction Full-duplex Both peers send anytime Schemes ws, wssPlain / TLS-encrypted Default Ports 80 / 443HTTP / HTTPS aligned Framing Binary frames Text (UTF-8) or binary payloads Max Frame 2^63 bytes 64-bit extended length Sub-protocol Sec-WebSocket-ProtocolApplication-level protocol
2. Understanding Handshake Process
Header Direction Purpose Upgrade: websocketClient Request protocol switch Connection: UpgradeClient Signal upgrade intent Sec-WebSocket-KeyClient Random 16-byte base64 nonce Sec-WebSocket-Version: 13Client Protocol version Sec-WebSocket-ProtocolBoth Subprotocol negotiation Sec-WebSocket-ExtensionsBoth Extension negotiation (e.g. permessage-deflate) Sec-WebSocket-AcceptServer SHA-1(key + GUID) base64 101 Switching ProtocolsServer Handshake success status
Example: Raw Handshake
GET /chat HTTP / 1.1 Host : example.com Upgrade : websocket Connection : Upgrade Sec-WebSocket-Key : dGhlIHNhbXBsZSBub25jZQ== Sec-WebSocket-Version : 13 Sec-WebSocket-Protocol : chat.v2 Origin : https://app.example.com HTTP / 1.1 101 Switching Protocols Upgrade : websocket Connection : Upgrade Sec-WebSocket-Accept : s3pPLMBiTxaQ9kYGzzhZRbK+xOo= Sec-WebSocket-Protocol : chat.v2
3. Understanding Frame Structure
Field Bits Description FIN 1 Final fragment flag RSV1-3 3 Reserved (used by extensions) Opcode 4 0x0 cont, 0x1 text, 0x2 binary, 0x8 close, 0x9 ping, 0xA pong MASK 1 1 for client→server (required) Payload len 7 / 7+16 / 7+64 Length encoding Masking-key 0 / 32 XOR key when MASK=1 Payload variable Application data
4. Understanding Connection States
Constant Value Meaning CONNECTING0 Handshake in progress OPEN1 Ready to send/receive CLOSING2 Close frame sent, awaiting response CLOSED3 Underlying TCP closed
5. Comparing WebSocket vs HTTP
Aspect WebSocket HTTP/1.1 Direction Bidirectional Client → Server Connection Persistent Request/response Overhead 2-14 bytes/frame ~500+ bytes/header Latency Low (no handshake per msg) Higher Caching No Yes Use Case Realtime Document/API fetch
6. Comparing WebSocket vs SSE
Feature WebSocket SSE Direction Bidirectional Server → Client only Protocol ws/wss (TCP) HTTP Binary Native Text only (base64 workaround) Auto-reconnect Manual Built-in Proxy support May require config Works through any HTTP proxy Max connections High Limited per origin (HTTP/1.1: ~6)
7. Comparing WebSocket vs Long Polling
Aspect WebSocket Long Polling Connections 1 persistent Repeated HTTP Latency ~ms Network RTT per msg Server load Low High (connection churn) Header overhead Minimal Full headers each request Implementation Native API Trivial fallback
8. Understanding Use Cases
Use Case Why WebSocket Chat / Messaging Bidirectional, low latency Live trading / quotes Sub-second updates, server push Multiplayer games Low overhead, binary frames Collaborative editing CRDT/OT delta sync IoT telemetry Persistent device→server channel Live dashboards Server-initiated updates WebRTC signaling Out-of-band SDP/ICE exchange
9. Understanding Protocol Limitations
Limitation Detail No request/response Must be modeled at app level No HTTP semantics No caching, no status codes per msg No multiplexing One logical channel per connection (HTTP/2 push N/A) Stateful Hard to scale horizontally without sticky sessions/pub-sub Proxy traversal Some intermediaries strip Upgrade headers Reconnection Manual logic required
10. Understanding Browser Support
Feature Support Notes WebSocket API All Browsers Chrome 16+, FF 11+, Safari 7+, Edge 12+ Binary (ArrayBuffer/Blob) All Browsers Set binaryType permessage-deflate Modern Browsers Negotiated; Safari has quirks WebSocketStream EXPERIMENTAL Experimental Chrome behind flag Workers/Service Workers All Browsers Service Workers cannot hold open WS